In the electromechanical arts, a switch is a device used for effecting the completion and interruption of a circuit. See "The Way Things Work: an Illustrated Encyclopedia of Technology", volume 1 published by Simon and Schuster, 1967, pages 96-97. There are various types of switches and switch configurations for various applications.
One type of switch is a pressure switch. In general, a pressure switch has an upper electrode and a lower electrode spaced from each other. Under a predetermined (threshold) load, the two electrodes are brought together and make contact. The switch has two positions (states), open and closed. Typically the switch is open when an applied load is less than the threshold pressure such that the two electrodes are spaced apart from each other. The switch is in its closed state when a load greater than the threshold pressure is applied to the switch causing the two electrodes to make contact with each other.